The present invention relates to an apparatus for correcting a vertical aperture of an image for use in improving a sharpness of an image.
Electronic still cameras that can record color photographs according to a magnetic recording system instead of a metal halide photograph have the following specific features:
(1) Conventional techniques, such as a VTR (video tape recorder), a floppy disk or the like can be applied to the electronic still camera; and PA1 (2) Electronic still cameras can record an image on an inexpensive magnetic recording medium. PA1 (1) A matrix correction cannot be effected and a color moire in the vertical direction is increased as is described in Japanese laid-open patent publication No. (JP-A-)1-143482; and PA1 (2) A resolution in the vertical direction becomes insufficient and a still picture of a satisfactory image quality cannot be obtained. For this reason, it is desired that a still picture is recorded according to the frame still picture recording system. However, when a still picture is recorded according to the frame still picture recording system, if the cameraman takes a picture of an object whose motion is large, there is then the problem that an image is blurred to cause an image to become a double-image.
Therefore, electronic still cameras are expected as one of promising future media. The Journal of the Institute of Television Engineers of Japan (Vol. 39, No. 9 (1985), pp. 760 to 764) describes all details of the electronic still camera.
As an imaging device utilized to realize an electronic still camera, it is to be desired that an imaging device of a so-called pixel mixing system that is generally utilized in the existing video cameras is used because the imaging device of the pixel mixing system can be utilized for general purpose and conventional video camera techniques can be applied to the imaging device of the pixel mixing system.
A CCD (charge-coupled device) imaging device of a pixel mixing system mixes and reads out adjacent two pixels of upper and lower lines within the imaging device in order to reduce a blur produced on an image by the motion of an object.
FIG. 2 of the accompanying drawings shows a typical example of a filter array of the conventional CCD imaging device of the pixel mixing system. As shown in FIG. 2, in the first field, pixels of adjacent upper and lower two lines are read out at every horizontal scanning as A1, A2, A1, A2, . . . , and pixel signals of upper and lower lines are added and then output at every pixel. Similarly, in the next field, pixels of upper and lower two lines are added and then read out at every horizontal scanning as B1, B2, B1, B2, . . . As a result, the CCD imaging device of the pixel mixing system alternately outputs (Mg+Ye) and (G+Cy) or (G+Ye) and (Mg+Cy) at every horizontal scanning. Assuming now that Wr=Mg+Ye, Gb=G+Cy, Gr=G+Ye and Wb=Mg+Cy, then a signal output from the imaging device is expressed as shown in FIG. 3 where S(n) represents an original signal, S(n-1) represents a signal delayed by 1H (H is a horizontal scanning period) and S(n-2) represents a signal delayed by 2H.
An aperture correcting circuit is adapted to correct a spatial frequency deteriorated by some optical elements, such as a lens and a quartz filter. As the aperture correcting circuit, there are known a horizontal aperture correcting circuit that can improve a sharpness of an image in the horizontal direction and a vertical aperture correcting circuit that can improve a sharpness of an image in the vertical direction.
FIG. 4 shows a vertical aperture correcting circuit as an example of a vertical aperture correcting circuit utilized in a conventional color video camera. This vertical aperture correcting circuit is described in ["Image Electronic Circuit" in "Image Electronics Course" edited by the Institute of Television Engineers of Japan, pp. 97-100] published by Corona Publishing Co.
In FIG. 4, reference numeral 13 depicts an imaging device, 14, 15 1H delay circuits, 16, 18, 23 adding circuits, 19 a subtracting circuit, 17, 21 coefficient circuits, 20 an LPF (low-pass filter), 9 a horizontal aperture correction signal generating circuit and 10 a low band luminance signal generating circuit, respectively.
As shown in FIG. 4, the video signal that was earlier described with reference to FIGS. 2 and 3 is output from the imaging device 13. The output video signal is delayed by 1H (i.e., one horizontal scanning period) by the 1H delay circuit 14 and further delayed by 1H by the 1H delay circuit 15 so that the 1H delay circuit 15 outputs a video signal delayed by 2H (i.e., two horizontal scanning periods).
The adding circuit 16 adds the original signal from the imaging device 13 and the 2H delay signal from the 1H delay circuit 15. The added signal from the adding circuit 16 is multiplied with a coefficient of 1/2 by the coefficient circuit 17 and then supplied to the adding circuit 18 and the subtracting circuit 19.
The subtracting circuit 19 subtracts the output of the coefficient circuit 17 from the 1H delay signal from the 1H delay circuit 14. Consequently, the subtracted output from the subtracting circuit 19 becomes a signal that has a positive or negative peak at the portion where an image is changed in the vertical direction, i.e., a signal indicative of a vertical aperture (contour) portion. The output from the subtracting circuit 19 is eliminated in frequency component of the horizontal direction by the low-pass filter (LPF) 20 and multiplied with a gain coefficient K by the coefficient circuit 21, thereby being adjusted in gain. Thus, a vertical aperture correction signal YV is generated from the coefficient circuit 21.
The vertical aperture correction signal YV is expressed by the following equation (1): EQU YV=-S(n)/4+S(n-1)/2-S(n-2)/4 (1)
where S(n) is the original signal, S(n-1) is the 1H delay signal and S(n-2) is the 2H delay signal.
Accordingly, the vertical aperture correction signal generating circuit 22 becomes a three-order FIR (finite impulse response) filter having a frequency characteristic shown in FIG. 5. As is clear from FIG. 5, assuming that fs is a sampling frequency of the vertical direction, then a frequency component near fs/2 of the frequency components of the vertical direction is emphasized and added by the adding circuit 16 and subtracted in opposite phase by the subtracting circuit 19. As a consequence, the frequency component having a frequency near fs/2 has the largest amplitude and this amplitude becomes small as the frequency thereof is away from the frequency fs/2.
On the other hand, the adding circuit 18 adds the output signal from the coefficient circuit 17 and the 1H delay signal from the 1H delay circuit 14 to generate an averaging signal in the vertical direction. Therefore, the output signal from the adding circuit 18 becomes a signal blurred in the vertical direction.
Since the horizontal aperture correction signal generating circuit 9 has a high-pass filter (HPF) action to emphasize a high band frequency component of the output from the adding circuit 18, the horizontal aperture correction signal generating circuit 9 generates a horizontal aperture correction signal YH in which a contour portion in the horizontal direction is emphasized.
The low band luminance signal generating circuit 10 has a low-pass filter (LPF) action to reduce a high band frequency component of the output from the adding circuit 18. Consequently, the low band luminance signal generating circuit 10 generates a base luminance signal YL which is blurred both in the vertical and horizontal directions.
The adding circuit 23 corrects the base luminance signal YL by using the horizontal aperture correction signal YH and the vertical aperture correction signal YV to output a desired luminance signal Y.
In the imaging device of the pixel mixing system used in the conventional color video cameras, as shown in FIG. 1, pixel signals of the upper and lower two lines are added within the imaging device and then output. In this case, three primary color components R (red), G (green), B (blue) of a light contained in lights obtained from the respective pixels are expressed as: ##EQU1##
Therefore, assuming that Y(n-1) represents a luminance signal spectral characteristic of the 1H delay signal, then the luminance signal spectral characteristic in the horizontal scanning is expressed by a sum of adjacent pixels. Thus, the luminance signal spectral characteristic Y(n-1) is expressed as: EQU Y(n-1)=Wr+Gb=2R+3G+2B (3)
Further, assuming that Y(n) represents a luminance signal spectral characteristic of the original signal and the 2H delay signal, then the luminance signal spectral characteristic Y(n) is expressed as; EQU Y(n)=Gr+Wb=2R+3G+2B (4)
Therefore, the luminance signal spectral characteristics of the respective lines become equal to each other.
In order to record a still picture by using the imaging device of the pixel mixing system, there are generally used a frame still picture recording system in which output signals of respective fields in which pixels are mixed are interleaved to record an image of one frame and a field still picture recording system in which only an image of one field is recorded. The field still picture recording system suffers from the following disadvantages;
To solve the aforesaid problems, unlike the aforementioned prior art in which the pixels are mixed as A1, A2, A1, A2, . . . , within the imaging device at every line in each horizontal scanning in the first field as earlier noted with reference to FIG. 2, there is considered a still picture recording system in which pixel signals are read out as A1, A2, A3, . . . in the first field and pixel signals are read out as B1, B2, B3, . . . in the next field to thereby record a still picture as shown in FIG. 6. In this case, the signal of each pixel on the imaging device holds an image at a certain timing point, and an incident light is shielded until the read-out of pixels of two fields is ended, whereafter the incident light is permitted to enter the imaging device one more time and then an object is picked up. Therefore, according to this still picture recording system, a blur of an image due to the motion of an object can be avoided, whereby a still picture can be recorded with less deterioration of a resolution. This still picture recording system will hereinafter be referred to as a full frame still picture recording system.
It was understood that the technique that pixels from the imaging device of the pixel mixing system are not mixed and read out can be realized even by a commercially available imaging device under the control of a driving pulse supplied to the imaging device. However, to realize the full frame still picture recording system, a video signal must be sequentially read out by 1H each in the order of color filters arrayed on the imaging device at the unit of pixels. Image data forming one frame picture is output from the imaging device at every field in every other line and cannot be processed in a predetermined signal processing fashion under this condition. After the examination, it is concluded that, if a frame memory that can store image data of two fields (one frame) output from the imaging device is employed and the output signal from the imaging device is read out from the frame memory in a non-interlace fashion after the output signal of one frame from the imaging device had been stored in the frame memory, then a still picture can be recorded according to the full frame still picture recording system. Study of examined results of image simulation reveals that, even when a still picture is recorded by using the imaging device of the pixel mixing system according to the full frame still picture recording system, an image with a sufficient image quality can be obtained as a still picture. However, since the luminance signal spectral characteristics of respective horizontal lines output from the imaging device are not coincident, the conventional vertical aperture correction processing (vertical enhancing processing) is not effective and as result, a line pair component occurs in the picture. This problem will be described below.
According to the aforesaid full frame still picture recording method, the imaging device in which color filters are arrayed as shown in FIG. 6 alternately outputs Mg and G in the horizontal scanning of a certain field. In the horizontal scanning of the next field, the imaging device alternately outputs Ye and Cy, which are then stored in a frame memory. The frame memory alternately outputs Mg and G in the horizontal scanning of a certain field and also alternately outputs G and Mg in the horizontal scanning of the next field. In this case, the output signal is presented as shown in FIG. 6 where S(n) represents the original signal, S(n-1) represents the 1H delay signal and S(n-2) represents the 2H delay signal.
If the pixel signals adjacent in the horizontal direction are added and then output in FIG. 6, then a luminance signal spectral characteristic Y(n-1)' of the 1H delay signal S(n-1) is expressed as: EQU Y(n-1)'=Ye+Cy=R+2G+B (5)
Then, a luminance signal spectral characteristic Y(n)' of the original signal S(n) and the 2H delay signal S(n-2) is expressed as: EQU Y(n)'=G+Mg=R+G+B (6)
Accordingly, the luminance signal spectral characteristics become different values at every horizontal scanning. As a result, the luminance signal has a frequency component of fs/2 (fs is the sampling frequency in the vertical direction) in the vertical direction. Also, since the conventional vertical aperture correcting circuit shown in FIG. 4 has the frequency characteristic shown in FIG. 5, the frequency component fs/2 is superimposed upon the vertical aperture correction signal as it is. Consequently, the line pair component occurs in the picture, which unavoidably leads to the deterioration of the image quality of the still picture.